When producing lightweight granulate sodium tripolyphosphate, there has hitherto been used a method in which so-called ortomix-powder is added with water (about 10%) in a schugi-mixer through spray nozzles, while vigorously agitating the mixer contents with the aid of knives arranged therein. The powderous product is agglomerated to a certain extent as a result of this mixing process.
The material leaving the schugi-mixer is introduced into a rotating calcining drum (1.5 rpm) in which the water is driven-off at a temperature of about 500.degree. C., thereby to form sodium tripolyphosphate in the form of lightweight granules. The material hitherto obtained has had a bulk entity of 0.25-0.59 kg/l and 42% of the granular material has been found to lie beneath 0.25 mm, when passed through a screen. The ortomix-powder used is prepared by mixing disodium phosphate solution and sodium dihydrophosphate solution in a molar ratio of 2:1, and spray drying the resultant solution to form an ortomix-powder.
The quantity of fine material present can be reduced by increasing the amount of water charged to the schugi by 11-12%, although the bulk density will then exceed permitted specifications and require the schugi to be flushed clean more often than would otherwise be the case.
The lightweight granular sodium tripolyphosphate is admixed with washing-detergent compositions containing additional tensides and carriers. In this respect, it is important that as far as possible all the products used have the same bulk density and particle-size distribution, so as to avoid separation of the products one from the other, i.e. to prevent a product from separating-out in the mixture as a result of vibration and, for example, becoming concentrated in the bottom of the packet containing the composition.
It is readily obvious that a separation of the various constituents one from the other will negatively affect the quality of the final product and its usefulness. A poor quality can lead to lost markets, which are difficult to recover.
There is consequently a need for a product which is lighter in weight than the known product, and in which the bulk-density ranges are narrower between produced batches, and have a particle-size distribution such that the percentage of particles smaller than 0.25 mm does not exceed 35%.